(and basically unplanned beyond what you see here)
Tokyo SOS ends - we are shown a vial containing Godzilla Cells
An ambitious scientist, on the payroll of a rogue government within Japan, takes possession of the G-Cells and, through the use of some cobbled together mish-mash of found alien technology, and advanced Earth-like technology, he begins the process of growing Godzilla embryo, each one held within a synthetic egg shell, with egg-ish stuff inside upon which they feed and grow.
After rapidly growing one of these to maturity, and unleashing it upon an unsuspecting public in some random Japanese city (to be determined if I ever decide to continue with this) the creature destroys and kills freely for an hour or so, before it expires and falls dead to the ground. This being a test, the monster was engineered with a limited life span.
The rampage is filmed from assorted camera vehicles (cars and trucks at ground level, helicopters and planes from the air) as well as from several rooftops in the city, the video feeds from which are streamed out across the internet, through a secure feed to various governments or shady-types, and the bidding commences as countries vie for ownership of their own Godzilla like creature.
Eventually, all of the "eggs" are sold (a total of ten which remain after the "test" monster), and are delivered to the assorted buyers, along with the equipment required to bring them to maturity and maintain control of their creature. At the end of all this, once the creatures are matured, there are ten Godzillas in the world.**
These "Godzillas" are unleashed in the theater of various existing wars, as well as set free upon the enemies of whatever country owns them. Mass destruction ensues (it looks really cool).
Unfortunately, beneath the ocean, the "real" Godzilla, having been taken to the bottom of the ocean by Kiryu in Tokyo: SOS, awakens, till held in the arms of its destroyer, and wrapped in the remnants of the Mothra Silk, and, after freeing itself, returns to the surface having been drawn back by the emergence of so many "relatives", in much the same manner as Godzilla recognizes Kiryu in Godzilla Against Mecha-Godzilla (and vice versa).
Of course, as this happens, all the other Godzillas are awakened to the presence of their progenitor and, breaking the shackles of control, all begin the varied treks from whatever point in the world they happen to be, back to Tokyo where Godzilla (the original) is waiting.
Unlike with Kiryu, however, these Godzillas do not "team-up" with Godzilla in an orgy of destruction, but instead, upon meeting in all the same place, are determined to wipe out all others and claim the throne as the Greatest Godzilla EVER, by destroying all others.
A huge fight between all the monsters ensues, and a whole lot of stuff is destroyed, people are killed, planes are crashed, cars are pooped on by pigeons...Basically, bad stuff for humans happens, until one Godzilla stands victorious.**
** Each of the Godzillas is one of the previous incarnations seen throughout the years of Godzilla movies (54, 84, 91, etc) and the final victor has been previously decided through online voting held by Toho, not to be revealed until the actual movie is released.